pgsql: doc: Improve some markups and some wording around archiving modu

doc: Improve some markups and some wording around archiving modules

This commit adds or fixes used markups in a couple of places in the docs
(for <command>, <systemitem> and <literal>). While on it, clarify some
of the documentation added recently for archiving modules with
archive_command, that would still be used as default choice if no
external module is defined (though an archive module could as well use
an archive_command).

Author: Maxim Yablokov
Discussion: /messages/by-id/b47ec4e8-6f6a-2aba-038e-d5db150b245e@postgrespro.ru
Backpatch-through: 15

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4afe79bd31e82b1b2d725c463a4aff2f85d8a3d0

Modified Files
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doc/src/sgml/adminpack.sgml | 7 ++++---
doc/src/sgml/archive-modules.sgml | 6 +++---
doc/src/sgml/backup.sgml | 23 +++++++++++++----------
doc/src/sgml/basebackup-to-shell.sgml | 8 ++++----
doc/src/sgml/ref/alter_publication.sgml | 2 +-
doc/src/sgml/ref/pg_basebackup.sgml | 4 +---
doc/src/sgml/ref/psql-ref.sgml | 10 +++++-----
7 files changed, 31 insertions(+), 29 deletions(-)